I think it's that the series so far has two colours in its emotional palate: silent brooding and outright misery.
Compare this to Band of Brothers. Sure, it covered the "war is hell" angle all the way through, and showed us the horrifying psychological effects it had on the soldiers. But there was also a certain amount of joy, found in friendship and camaraderie, to be had. Winters and Nixon had a solid, well-developed friendship. We saw the men of Easy Company become a family--it's right there in the title. Characters like Garnier, Bull and the rest of the guys were fun to get to know. Watching it you don't envy the crap they go through, but you do envy the brotherhood they developed. It was developed, strong and consistent throughout the show.
In The Pacific, we just have three brooding, joyless, loner-type men who get more and more miserable as the series progresses. They aren't characters who you'd want to share a drink with. Basilone and Sledge barely have personalities, they're more archetypes than anything: the "war hero guilty about being away from the war" and "the boy who loses his innocence". Leckie is a little bit more rounded, but I find him an unlikeable jerk. It's hard to care about their situations because they tend to do two things: mope about their crappy lives and look scared when things are blowing up.
What's missing is the brotherhood. Without that, this war is just an unending hell.